IP Aggregation

What is IP Aggregation???

IP aggregation or Supernetting or Summarization is used to summarize a number of subnets into one big supernet. It serves as a critical tool which will use to reduce the size of routing table and helps reducing the processing power and memory usage of the router.Aggregation can be lethal in reducing the size of routing table which may become a big stability factor in use of link state routing protocols like OSPF and ISIS.

Supernetting was introduced to solve the problem with rapidly growing routing tables and because of IP version 4 address pool exhaustion. The main benefits of route aggregation are less memory and CPU power requirements for storing and processing routes. More information about supernets can be found in RFC 1338 - Supernetting: an address assignment and aggregation strategy.

Now let's have a look how to find supernet address if we are given subnets addresses. Actually to calculate the supernet IP we need minimum two IP addresses. You'll understand what I told in here when we finish calculating the supernet IP address.

So let's consider following subnets addresses.
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.4.0/24

So by looking this we can't see or guess the supernet IP address. Why? We can't say that these are the only subnets addresses that exists in that supernet. If we are given that it is easy to find the supernet address. So in here I assume that those subnets addresses are a set of subnets of the supernet.

so to find supernet IP, follow given instructions.

• write the bit pattern of all given IP's and align them.

11000000101010000000000100000000

11000000101010000000001000000000

11000000101010000000001100000000

11000000101010000000010000000000

• find the common bit pattern of those bit patterns

110000001010100000000

• count how many bits are there for that common bit part. This will be a prefix of those IP addresses

110000001010100000000 - 21 bits

• fill remaining bits with zeros and convert to dotted decimal pattern. Use number of bits to show the IP address in CIDR notation.

11000000101010000000000000000000 >>> 192.168.0.0/21

So it is easy to find the supernet IP address if you follow those steps.